Music is now so readily accessible in digital form that personal collections can easily exceed the practical limits on the time we have to listen to them: ten thousand music tracks on a personal music device have a total duration of approximately 30 days of continuous audio. Distribution of new music recordings has become easier, prompting a huge increase in the amount of new music that is available. In 2005, there was a three-fold growth in
legal music downloads and mobile phone ring tones, worth $1.1 billion worldwide, offsetting the global decline in CD sales; and in 2007, music downloads in the U.K.
reached new highs [1]–[3].
Traditional ways of listening to music, and methods for discovering music, such as radio broadcasts and record stores, are being replaced by personalized ways to hear and learn about music. For example, the advent of social net-working Web sites, such as those reported in [4] and [5],has prompted a rapid uptake of new channels of music discovery among online communities, changing the nature of music dissemination and forcing the major record labels to rethink their strategies.
